/Engine/Source/Runtime/Core/Public/Serialization/PackageWriter.h
Returns an AssetRegistry describing the previous cook results. This doesn't mean a cook saved off to another directory - it means the AssetRegistry that's living in the directory we are about to cook in to.
[TUniquePtr](API\Runtime\Core\Templates\TUniquePtr)< [FAssetRegistryState](API\Runtime\AssetRegistry\AssetRegistry\FAssetRegistryState) > LoadPreviousAssetRegistry()